A plurality of ceramic green sheets are used in the manufacture of multilayer ceramic substrates for integrated circuit semiconductor package structures. Via holes are punched in the ceramic green sheets to form a path for electrical interconnections through the sheets. The step of punching the via holes in ceramic green sheets presents formidable engineering problems in view of the small size and density of the holes and of the complex hole patterns needed. The sheets themselves are typically thin: only about 0.2 mm (8 mils) thick.
It is convenient to punch via holes with a tool of the type disclosed in U.S. Pat. No. 4,425,829 issued to Kranik et al. In this type of tool, a plurality of punch elements are arranged in a grid on a punch head and are indexed over the green sheet which is covered by an interposer mask. The interposer mask contains openings where holes will be punched. When the punch elements contact the interposer mask, as the punch head is moved downwardly, a hole will be punched where the openings occur because the punch element will pass through the openings in the interposer mask and then through the ceramic green sheet. In other areas covered by the interposer mask (i.e., where holes are not desired), the interposer mask will cause the punch element to be retracted into the head. The green sheet is sequentially indexed through a predetermined number of positions to complete the punching of a sheet.
It is essential that the punching operation produce products free from defects. A single defect can potentially render a green sheet unsuitable for further processing. It is also essential that the punching operation be rapidly and accurately performed. Each green sheet can contain over 100,000 punched holes. Of particular concern is the adherence to the tip of the punch of a slug punched from the sheet. The inherent adhesion characteristics of the unfired green sheet are amplified by the large punching force applied over the small area of the punch tip. The diameter of the punch tip can be as small as 0.13 to 0.15 mm (5 to 6 mils) in current application and is expected to be 0.10 mm (4 mils) or less for advanced substrates, resulting in a pressure at the punch tip on the order of 2,700 kg/cm.sup.2. If the punch slug adheres to the punch, the slug may be drawn back into the punched hole, causing a substrate defect. To eliminate the likelihood of such defects, it has been standard practice to use two punch strokes for each hole. This practice greatly increases green sheet processing time.
The problem of slug adhesion to the punch is not limited to the punching of ceramic green sheets; rather, the problem has been discussed in other punching application references. One method used in punching apparatus for the removal of punch slugs is the use of either pressurized air or a vacuum to force the slug from the punch. Certain references disclose apparatus in which air is channeled through the punch to remove the slug from the tip of the punch. This method is not practical, however, for punching extremely small diameter holes. Other applications either direct air into or apply a vacuum to a chamber below the punch to clear the slugs and do not directly address the problem of slug adherence.
The use of air flow slug removal methods in ceramic green sheet punching to achieve single stroke punching is disclosed in U.S. Pat. No. 5,111,723 issued to Andrusch et al. and U.S. Pat. No. 4,425,829 issued to Kranik et al. Kranik et al. teach a tube protruding into the die bushing which upwardly injects air into the die cavity below the punching area. This air flow induces circulation in the die bushing cavity which assists in forcibly removing slugs from the punch. The arrangement does not provide the repeatability necessary to achieve single stroke punching.
Andrusch et al. teach a single stroke punch apparatus which includes a punch and a bushing retention die plate. A support bushing is mounted in the die plate and provides support for the workpiece. The support bushing has a clearance for a punch. The apparatus also has a nozzle (or "slug removal bushing") mounted in the die plate which provides an internal passage for the removal of punch slugs from the apparatus. A slug is punched from the workpiece through an opening in an end wall of the support bushing disposed in an aperture of the die plate. The nozzle, the support bushing, and the bushing retention die plate define a flow passage allowing gas to flow in the die plate to the opening in the support bushing. The gas flow impinges on the slug attached to the punch tip proximate to the end wall of the support bushing and at the top of the nozzle to remove the slug from the punch through a slug removal passage in the nozzle.
The flow passage includes a channel (or "slot clearance") between the end wall at the top of the nozzle and the support bushing. Unfortunately, the slot clearance prevents a sealed surface between the support bushing and the nozzle, thus allowing the rapid expansion of gas (air) as it enters the region immediately below this interface. Without a sealed interface, the gas tends to expand too quickly into the volume surrounding the punch tip and slug and is sometimes ineffective for blowing the slug from the punch tip.
In addition, the gas flow impinges on the slug attached to the punch tip just after the punch tip clears the end wall of the nozzle--where the punch tip minimally protrudes through the support bushing. The punch is moving downward rather quickly at this point in its reciprocating travel path and, therefore, the impingement of the gas flow on the slug is almost instantaneous. The duration of impingement is sometimes insufficient to blow the slug from the punch tip.
Finally, the position of a slot clearance between the top of the nozzle and the support bushing facilitates wear of the nozzle. Such wear reduces the efficiency of the punch over time. This position of the slot clearance also renders the diameter of the gas flow inlet dependent on how well the support bushing fits on the top of the nozzle. Consequently, the effective inlet diameter is variable.
Although both the Kranik et al. and Andrusch et al. configurations have been useful in removing slugs from the punch tip, neither configuration provides the 100% slug removal which is required for single stroke punching. A 99.9% slug removal rate on a green sheet containing 100,000 holes results in 100 defects per sheet, any one of which renders the sheet unacceptable for further processing. The problem can be further appreciated by considering that a defect may not be detected until the green sheet is laminated into a substrate containing 60 or more layers. Thus, the conventional tools used to remove a punch slug are not effective to the degree needed when punching 0.2 mm (8 mils) thick green sheets in high volume manufacturing. The slug that is punched out is not always removed, resulting in via blockage.
The deficiencies of the conventional punch apparatus show that a need still exists for a tool which will precisely direct an air flow at a punch tip to remove the punch slug and thus allow single stroke punching. To overcome the shortcomings of the conventional punch apparatus, a new punch tool is provided. An object of the present invention is to provide a single stroke punch tool which offers improved punch slug removal.
A related object is to improve the design of the converging-diverging nozzle that fits within the die bushing of the punch tool to achieve air flow velocities sufficient to remove the slug from the punch tip. It is a further object of the invention to provide a punch slug removal tool which precisely directs an air flow at the slug adhered to a punch tip. Another object is to assure that the maximum force of the air flow is applied to the adhered slug for the maximum length of time, thus making the air flow more effective.
It is still another object of the present invention to seal the surface between the support bushing and the nozzle, preventing the rapid expansion of the gas which reduces the effectiveness of the gas for blowing off the slug. An additional object is to fix the effective inlet diameter of the path for the gas flow. Yet another object of this invention is to eliminate wear of the nozzle, thereby improving the efficiency of the punch over time.